What are the prerequisite to create a person account in salesforce?

Please ensure the record type is made available to all profiles that need access to it's functionality. To confirm the record type is assigned for the profiles, follow these steps:
1. Click on Your Name ; Setup ; Admin Setup ; Manage Users ; Profiles.
2. Click on the name of a specific profile, and scroll down to the "Account Record Type Defaults" section to select values for Business or Person Accounts. Or, if you have the new Enhanced Profile version enabled, then Object Settings > Accounts.
3. User profiles that have read access to accounts have read access to contacts. The organization-wide default sharing is set so that "contacts" are controlled by "parents" or "accounts" and "contacts" are both "private".
4. From Setup, enter Account Settings in the Quick Find box, and then select Allow Customer Support to enable Person Accounts.After this you have to log a case for salesforce customer support to enable Person Account.